1. Technical Field
The present invention relates to a power supply unit including two secondary batteries: a lead secondary battery, and a second secondary battery (such as a lithium secondary battery) with higher power density and energy density than the lead secondary battery.
2. Description of the Related Art
A lead secondary battery that supplies power to various electrical loads, such as a starter motor, is commonly mounted in vehicles that use an internal combustion engine as a running drive source. Although the lead secondary battery is less expensive than high-output and high-energy-density secondary batteries (high-performance secondary batteries), such as nickel secondary batteries and lithium secondary batteries, its endurance against frequent charge and discharge (cumulative charge and discharge amount) is low. The lead secondary battery is frequently discharged particularly in vehicles having an idle-stop function, and early deterioration of the lead secondary battery becomes a concern. In addition, the lead secondary battery is frequently charged in vehicles having an alternator that charges the lead secondary battery by generating power from regenerative energy of the vehicle, and early deterioration of the lead secondary battery becomes a concern. Simply changing the lead secondary battery to the above-described high-performance battery to address these concerns leads to a significant increase in cost.
Therefore, JP-A-2007-46508, JP-A-2007-131134, JP-A-2008-29058, 3′P-A-2008-155814, and JP-A-2009-126395 propose to mount both the high-performance secondary battery (second secondary battery) having high endurance against frequent charge and discharge, and, the inexpensive lead secondary battery in parallel connection. In other words, the high-performance secondary battery preferentially supplies power to electrical loads and is charged (regenerative charge in particular) during idle-stop, thereby suppressing deterioration of the lead secondary battery. On the other hand, the inexpensive lead secondary battery supplies power required over a long period of time (dark current supply), such as when the vehicle is parked, thereby reducing the capacity of the high-performance secondary battery and suppressing increase in cost.
Early deterioration becomes a concern when the secondary battery is in an overcharged state or an over-discharged state. Therefore, charge and discharge of both secondary batteries are required to be controlled such that a state of charge (SOC: percentage of actual amount of charge in relation to amount of charge at full charge) expressing the charging state is within a range (appropriate range) where the secondary battery is not overcharged or over-discharged. Specifically, when the SOC of the lead secondary battery falls below the appropriate range, charge of the lead secondary battery is promoted by a setting voltage Vreg of the generated power regulated by a regulator being set to a high voltage. On the other hand, when the SOC of the lead secondary battery rises above the appropriate range, discharge from the lead secondary battery is promoted by the setting voltage Vreg being set to a low voltage.
However, although the value of the open voltage of the secondary battery differs depending on the SOC, the open voltage (such as 12.7V to 12.8V) in the appropriate range of the lead secondary battery and the open voltage in the appropriate range of the high-performance secondary battery ordinarily do not match. Therefore, for example, when the setting voltage Vreg is set to a high voltage to promote charge of the lead secondary battery, the generated power flows into the high-performance secondary battery even when the SOC of the high-performance cell is sufficiently high, and overcharge of the high-performance secondary battery becomes a concern. Furthermore, power is supplied from the high-performance secondary battery to the electrical loads, such as the starter motor, disposed such as to operate using the lead secondary battery as the power supply source, and thus, over-discharge of the high-performance secondary battery becomes a concern.